Semiconductor devices include components that have characteristics that vary with respect to temperature. For example, as temperature increases mobility of charge carriers decreases, causing transistors such as insulated gate field effect transistors (IGFET) to have lower drive current. Although drive current decreases, leakage current (leakage current when the IGFET is turned off) increases. These temperature dependent characteristics can make design problematic.
When designing a semiconductor device, the designer will design circuit timing and internally regulated power supply voltages for worst case corners. Typically, a fast corner may be high voltage, low temperature and a slow corner may be low voltage and high temperature. By designing circuits in a semiconductor device for a worst case temperature, power may be unnecessarily wasted at another temperature point. For example, a power supply may provide a voltage that is unnecessarily high at a first temperature point due to the necessity of ensuring specifications are met at a second temperature point, even though the semiconductor device rarely operates at the second temperature point. This can cause power to be wasted at the first temperature point, which is where the semiconductor device typically operates.
A specific example is an internal refresh operation in a dynamic random access memory (DRAM). At a low temperature, charge on a DRAM capacitor in a DRAM memory cell may degrade more slowly than at high temperature. However, to ensure specifications are met, the frequency of refresh operations may be unnecessarily high at low temperatures to ensure the high temperature case is met. This can cause unnecessary power consumption in typical operating temperatures.
Unnecessary power consumption is even more important in mobile devices as it reduces battery lifetime.
In light of the above, it would be desirable to provide a semiconductor device in which parameters may be varied with respect to operating temperature.